1. Field of the Invention
The present invention generally relates to an image forming apparatus that forms a multicolor image by superposing single-color images.
2. Description of the Related Art
Some color image forming apparatuses, such as color printers, form a multicolor image by superposing single-color images of different colors. When forming an image with such an image forming apparatus, misalignment between superposed single-color images may occur because of various factors such as changes in ambient temperature in a space where the image forming apparatus is installed and changes in temperature inside of the image forming apparatus. The misalignment in turn causes changes in color of drawings and texts or non-uniformity (color irregularity) in images and thereby reduces their quality. Therefore, to obtain good image quality, it is necessary to align single-color images as accurately as possible.
To accurately align single color images, various mechanisms for correcting misalignment between single-color images are proposed for use in an image forming apparatus that forms single color images using, for example, multiple photoconductor units and then superposes the single-color images to form a multicolor image.
Japanese Patent No. 3569392 (patent document 1) discloses a deflecting mirror control device that makes it possible to accurately correct misalignment between toner images of different colors and thereby to form a high-quality image. In addition, the disclosed device includes a mechanism to report a malfunction in an image forming apparatus so that the image forming apparatus can be repaired before misalignment between toner images increases and image quality is greatly degraded.
Japanese Patent Application Publication No. 2000-112205 (patent document 2) discloses a color image forming apparatus including a magnification adjusting unit for correcting magnification of a reference color image. With the magnification adjusting unit, the disclosed apparatus can accurately correct magnification of single-color images including the reference color image. Japanese Patent Application Publication No. 2004-295083 (patent document 3) discloses an image forming apparatus and an image forming method that make it possible to easily correct a magnification error of an image and any irregularity at the edge of an image for each deflection surface even if these problems change over time and thereby make it possible to form a high-quality image.
As a common method used in patent documents 1 through 3, misalignment correction patterns formed on a transfer belt are detected using multiple sensors and misalignment between single-color images is corrected based on signals from the sensors.    [Patent document 1] Japanese Patent No. 3569392    [Patent document 2] Japanese Patent Application Publication No. 2000-112205    [Patent document 3] Japanese Patent Application Publication No. 2004-295083
In the above method of correcting misalignment between single-color images, the accuracy of the correction improves as the number of misalignment correction patterns to be detected by each sensor increases. However, as the number of misalignment correction patterns increases, the time necessary to detect the patterns increases and, as a result, the time necessary to correct the misalignment increases. Accordingly, this reduces the printing speed of an image forming apparatus.
Meanwhile, to correct misalignment while printing multiple images, it is necessary to form misalignment correction patterns between the images (or pages) on a transfer belt. In this case, since misalignment correction patterns are formed between pages, timings and areas for forming the misalignment correction patterns are restricted. Although the restriction can be eased by increasing the interval between pages on a transfer belt, it results in longer printing time and lower printing speed.